Electrolytic recording media



ELECTROLYTIC RECORDlNG MEDIA Robert Doughty Richards, Beckenham,England, aslsigner to Muirhead & Co. Limited, Beckenham, Engand NoDrawing. Filed July 1'6, 1957, Ser. No. 672,123 Claims priority,application Great Britain Dec. 20, 1956 7 Claims. (Cl. 204-2) Thisinvention relates to electro-sensitive media of the kind used inelectrolyte recorders and picture telegraph receivers.

A well known medium of this kind consists of a paper base impregnatedwith a solution containing chemical compounds which produce adark-coloured stain on the passage of an electric current through thepaper, which is used in a moist condition.

A known type of impregnating solution for the paper comprises apolyhydroxyphenolic compound, such as catechol, together with a strongelectrolyte and possibly other elements, which may be added to producespecific recording characteristics.

In the recorder two electrodes are arranged in contact with oppositesurfaces of the paper and means are provided to move at least one of theelectrodes in such a manner as to provide a series of scanning linesadjacent to each other so that the whole area of the paper isprogressively scanned. One of the electrodes (the anodic electrode)contains a metal, such as iron, which will go into solution in theimpregnant under the infiuence of an electric current and the dissolvedmetal reacts with the chemical compounds in the impregnant to form adark-coloured, water-insoluble dye.

A well known type of recording paper containing catechol includes astrong electrolyte such as potassium chloride or nitrate. In such apaper the marking is of a dark purple colour and does not provide thefull degree of contrast whihc is desirable when recording pictorialmatter. In our copending patent application Serial No. 517,659 arecording medium of this type is de scribed which, in addition to thedye-forming compound and an electrolyte, contains an oxidizing agent,such as potassium or sodium chlorate. The addition of the chlorateproduces a blacker mark, which lacks the undesired purple tinge, andproduces a higher degree of contrast; furthermore, the paper may beintensified by heating to a low temperature after recording in orderfurther to increase the contrast in the recording.

Where potassium chloride or nitrate, or a mixture of the two, is used aselectrolyte only a small proportion of chlorate may be introducedbecause of the relatively low solubility of potassium salts. With someformulae, the addition of more than about 6% of a chlorate causes.precipitation so that crystals and wooly growths form on the paperduring storage. As an alternative, the patent specification referred todescribes an impregnating medium in which the electrolyte consistsentirely of a so dium compound or compounds which, as is well known, arehighly soluble. By using sodium chlorate as the oxidizing agent in sucha formula it is possible to include a very much larger amount, with theresult that a deep black mark and a high contrast range between whiteand black are produced. Furthermore, the marking may be intensified veryreadily at a low temperature. Both sodium nitrate and sodium chlorateare hygroscopic and this property is undesirable as the impregnatedpaper dries slowly. In moist conditions, i.e., during periods of dampweather, a paper containing a high proportion of these elements neverdries completely but remains limp and does not acquire the crispnesswhich is a desirable feature. Furthermore, the dye which forms themarking is able to spread through the fibres of the paper so that themarking loses its sharpness and clarity.

The invention consists of an electrolytic recording medium impregnatedwith an aqueous solution containing a polyhydroxyphenolic compound whichis water-soluble, substantially colourless in weak solution and capableof forming a dark-coloured water-insoluble dye with an appropriatemetal, together with an electrolyte, an oxidizing compound and acompound which is efilorescent to assist rapid drying of the paper.

The invention further consists of an electrolytic recording medium ofthe kind described in which the electrolyte consists of a compound whichis efilorescent, together with an oxidizing compound.

Various compounds may be added to the impregnating solution for thepurpose of the invention. Di-sodium hydrogen phosphate is efilorescentbut it increases the initial pH value of the solution and alsosuppresses the pH value during recording, thus inhibiting lakeformation; furthermore, it reduces the storage life of the paper andalters the recording characteristics.

Ammonium di hydrogen phosphate might also be used but it is found thatthe records deteriorate quite rapidly after recording.

It is preferred to use sodium sulphate, or a sodium sulphate hydrate,which does not appreciably alter the recording characteristics or thestorage life of the paper. Sodium sulphate decahydrate, Na SO .l0H O,for example, will lose water under almost any atmospheric conditions.

It is found that a recording medium containing sodium sulphate driesvery rapidly after recording and acquires a crispness which is verydesirable in handling. Spreading of the dye which forms the markings isprevented.

A suitable formula for an impregnating solution containing sodiumsulphate is as follows:

Parts by weight Sodium sulphate decahydrate 10 Sodium chlorate 7.5Sodium nitrate 4.5 Pyrocatechol 3.5 Citric acid 0.22. T hiourea 0.22Water to If a potassium compound is used as electrolyte in a formulasuch as that given above, then the amount of sodium chlorate must bereduced, preferably to less than 6 percent.

If an impregnating solution containing relatively large amounts ofsodium sulphate and sodium chlorate is used a further feature results.Papers to be used for recording must be thin and of fine texture inorder to have the desired low electrical resistance and uniformrecording characteristics. Such papers shrink considerably in drying butif the solid content of the solution is high then the fibres of thepaper are supported by these solids whilst drying, with the result thatshrinkage is reduced to a fraction of what it would otherwise be and asmoother surface results, which is free from wrinkling and cockling.

A formula containing larger amounts of sodium sulphate, sodium chlorateand electrolyte is:

Parts by weight Sodium sulphate decahydrate 15 Sodium chlorate 15 Sodiumnitrate l Pyrocatechol 5 Citric acid 0.2 Thiourea 0.2 Water to 100 8.6Sodium chlorate 8.6 Pyrocatechol 3.5 Citric acid 0.22 Thiourea 0.22Water to 100 The quantities of sodium sulphate and sodium chlorate maysafely be increased up to 20 percent in a formula. of this type.

. It will be understood that the formulae are only given by way ofexample. Many variations may be made Without departing from the scope ofthe invention.

We claim:

'1; An electrolytic recording medium for facsimile or photo-telegraphicreceivers comprising a base impregnated with an aqueous solutioncontaining a polyhydroxyphenolic compound which is Water-soluble,substantially colourless in weak solution and capable of forming adark-coloured water-insoluble lake with an appropriate metal, togetherwith an electrolyte, sodium chlorate to deepen the colour of therecording and more than 1% of solution weight of sodium sulphatecompound to promote rapid drying of the medium after recording.

. 2. An electrolytic recording medium, as claimed in claim l, in whichthe electrolyte comprises a potassium compound, the oxidizing compoundconsists of up to 6 percent of solution weight of sodium chlorate, andup to 10 percent of solution weight of sodium sulphate decahydrate.

3. An electrolytic recording medium, as claimed in claim 1, in which theelectrolyte consists of a sodium compound, the oxidizing agent consistsof up to 15 percent of solution weight of Sodium chlorate and up to 15percent of solution weight of sodium sulphate decahydrate.

4. An electrolytic recording medium for facsimile or photo-telegraphicreceivers comprising a base impregnated with an aqueous solutioncontaining a polyhydroxyphenolic compound selected from the classconsisting of catechol and its derivatives which is water-soluble,substantially colourless in weak solution and capable of forming adark-coloured water-insoluble lake with an appropriate metal, togetherwith sodium chlorate to deepen the colour of the recording, and anelectrolyte which is water-soluble, substantially colourless and morethan 1% of solution weight of efllorescent sodium sulphate.

5. An electrolytic recording medium, as claimed in claim 4, comprisingup to 10 percent of solution weight of pyrocatechol, up to 20 percent ofsolution weight of a sodium chlorate, and up to 20 percent of solutionweight of sodium sulphate decahydrate.

6. A recording medium electrolytically markable on passage of electriccurrent therethrough between a pair of metal electrodes, comprising aporous support impregnated with an aqueous solution of an electrolyte, apolyhydroxyphenolic compound forming a coloured lake with the metal ofone said electrode, an oxidant selected from the class consisting ofchlorate and perchlorate metal salts to deepen the colour of said lake,and more than 1% of solution weight efflorescent sodium sulphate topromote rapid drying of the medium after marking.

7. A recording medium electrolytically markable on passage of electriccurrent therethrough between a pair of metal electrodes, comprising aporous support impregnated with an aqueous solution containing apolyhydroxyphenolic com-pound forming a coloured lake with the metal ofone said electrode, an oxidant selected from the class consisting ofchlorate and perchlorate metal salts to deepen the colour of said lake,and at least 4% of solution weight of an electrolyte which isefliorescent sodium sulphate to promote rapid drying of the medium aftermarking.

References Cited in the tile of this patent UNITED STATES PATENTS2,485,678 Trib-ble Oct. 25, 1949 2,681,309 Kline June 15, 1954 2,686,858Kohn Aug. 17, 1954

1. AN ELECTROLYTIC RECORDING MEDIUM FOR FACSIMILE OR PHOTO-TELEGRAPHICRECEIVERS COMPRISING A BASE IMPREGNATED WITH AN AQUEOUS SOLUTIONCONTAINING A POLYHYDROXYPHENOLIC COMPOUND WHICH IS WATER-SOLUBLE,SUBSTANTIALLY COLOURLESS IN WEAK SOLUTION AND CAPABLE OF FORMING ADARK-COLOURED WATER-INSOLUBLE LAKE WITH AN APPROPRIATE METAL, TOGETHERWITH AN ELECTROLYTE, SODIUM CHLORATE TO DEEPED THE COLOUR OF THERECORDING AND MORE THAN 1% OF SOLUTION WEIGHT OF SODIUM SULPHATECOMPOUND TO PROMOTE RAPID DRYING OF THE MEDIUM AFTER RECORDING.